Audio synthesizers with an analog sound generation chain, especially those with analog oscillators, are regarded as highly sought-after. In response to commands received, e.g., via the Musical Instrument Digital Interface (MIDI) protocol, a digital controller converts the received “play a note” instruction to a numerical value, which is then sent to one or more digital to analog converters (DACs). The analog control voltages yielded by those DACs are then routed to a number of analog voltage controlled oscillators (VCOs), which generate electrical waves of desired frequencies and of various wave forms. In many cases, these are then mixed, altered by one or more voltage controlled filters (VCFs), and amplified or attenuated to the desired volume by voltage controlled amplifiers (VCAs). The VCAs and the VCFs may be reactive to the user's inputs and adjustments, which coactively help to determine the character of the generated sounds.
VCO output frequencies usually respond to the applied control voltage levels either linearly or, more typically, exponentially (which translates to a frequency rise of one octave for each volt rise in the input voltage). Moreover, VCOs and their control voltage generating circuits have to be finely tuned so that the generated sound has exactly the correct frequency. This requires substantial calibration work for each manufactured oscillator, and thus constitutes significant effort for the manufacturer. Also, due to the nature of analog circuitry, frequencies can drift in the course of operating the audio synthesizer, dependent on components' tolerances and on conditions like the circuit's temperature, the ambient temperature, humidity, and other environmental factors.
As a result, elaborate and often costly circuitry is included in analog audio synthesizers to compensate for the problems of frequency calibration and operational drift. Even so, some manufacturers advise customers to let their synthesizers “warm-up” for 15-30 minutes after they are switched on, so that they might reach “operational temperature.” However, enthusiasts know very well that, even after such a warm-up period, re-calibrating is typically necessary.